You certainly need brains to make the grade as a London cabbie, a study has shown.

Researchers who followed a group of trainee taxi drivers found that gaining "The Knowledge" can alter brain structure.

But this only occurred in those who successfully qualified after spending up to four years memorising London's 25,000 streets and myriad landmarks.

Scans revealed they had a greater volume of "grey matter" at the back of the hippocampus, a brain region known to play key roles in memory and spatial navigation.

Grey matter consists of the "bodies" of nerve cells and is where neural processing takes place.

Drivers who failed the demanding series of "Knowledge" exams did not show evidence of the same changes - and nor did a comparison group of non-taxi drivers.

Lead scientist Professor Eleanor Maguire, from the Wellcome Trust Centre for Neuroimaging at University College London, had previously found enlargement of the hippocampus in London cabbies.

The studies suggested that the brain may have changed to accommodate an internal "map" of London.

The new research, published in the journal Current Biology, goes further and raises questions about "nature versus nurture".

"By following the trainee taxi drivers over time as they acquired - or failed to acquire - The Knowledge, a uniquely challenging spatial memory task, we have seen directly and within individuals how the structure of the hippocampus can change with external stimulation," said Prof Maguire. "This offers encouragement for adults who want to learn new skills later in life."